Figure 1
(a) Andrey Sokolov/Argonne National Laboratory; (b) Peter Vorobieff/University of New Mexico

Figure 1: (a) Image of bacterial turbulence in a thin, free-standing (soaplike) liquid film. Individual bacteria are seen as short, dark rods, yellow arrows indicate the direction and magnitude of the bacterial flow. As a consequence of the collective behavior of the bacteria, vortices often emerge even in conditions in which laminar flow would be expected. The work of Dunkel et al. allows the separate measurement of the movement of the bacteria and the surrounding liquid. (b) False-color image of high-Reynolds-number hydrodynamic turbulence in a flowing soap film (containing no bacteria). The image visualizes variations in the film thickness. Turbulent motion arises because of the gravity force acting on the liquid, and, in conventional liquids, can only occur when inertial forces dominate viscosity